1. Field of the Invention
The present invention is for an apparatus for passively damping flexural blade vibration in turbo-machinery. The apparatus comprises at least one mechanical insert, embedded into the blade near its tip. Detailed strength design ensures survival of the insert under the high centripetal loads experienced at the blade tip. Blade flexural energy is dissipated by friction forces and rubbing between the insert and the blade. Surface contact forces are provided primarily by the centripetal force.
2. Description of the Related Art
Compressor sections within an axial flow turbine engine are based upon a series of rotor assemblies. Each rotor assembly comprises a rotating disk and a plurality of blades circumferentially disposed around the disk. The blades are either separate pieces, assembled to the disk, or the entire bladed disk is machined from a single piece of metal.
In operation, compressor blades are loaded centripetally and aerodynamically. The aerodynamic loading is nominally slowly time-varying, varying only with engine operating condition (rotor speed and stage mass flow and pressure rise). But blade vibration is often also excited by rapidly varying aerodynamic loading. Two principal mechanisms for this excitation are: 1/Self-excited flutter, and 2/Aerodynamically forced vibration, where the forcing source is flow inhomogeneities. If these flow inhomogeneities are circumferentially periodic, then the forced vibration may be resonant and some narrow ranges of rotor operating speed will resonantly excite such vibrations.
Blade vibration may occur in any of many natural modes of vibration of the blades. Lower-order modes are generally predictable and relatively easy to avoid exciting or to damp. Higher order modes are generally more difficult to predict and more difficult to damp. These plate-like modes are commonly excited at resonance by flow inhomogeneities created by airfoils in adjacent stages in the engine. These modes typically involve plate-like deformation patterns, with largest motions and largest flexural strains near the tip of the blade.
Left unchecked, blade vibration can cause premature blade failure and can liberate a portion of the blade, causing substantial damage to the engine. In either vibration case, (flutter or resonant forced vibration,) passive damping helps reduce the amplitude of the vibratory material stresses and thus extends the life of the blade.
The above-listed patents address damping of blade vibration in turbomachinery. With the exception of U.S. Pat. No. 5,498,137 (El-Aini et al.), all of these patents address low-order modes. Most of these patents describe a damping system which acts upon blade-to-blade motion at the root of the blades, motion which is not present in high-order modes.
Only the El-Aini et al. patent is closely related to the present invention. El-Aini et al. describe a pocket machined into a solid-section metal fan or compressor blade, a lid fastened over that packet and contoured to match the outer shape of the blade, and any of several different damping inserts placed into that pocket.
Like El-Aini et al., the present invention also addresses higher-order plate-like vibration modes of solid-section metal blades. Like El-Aini et al, the present invention also proposes a pocket machined into a surface of the blade, near the blade tip. But unlike El-Aini et al., the present invention does not propose a lid and a damping insert. Instead, the pocket is machined with under-cut edges, and a metal sheet is slipped into this pocket such that the undercut edges of the pocket engage the metal sheet and prevent it from escaping. The metal sheet itself forms the contoured aerodynamic surface of the blade.
Damping is achieved by rubbing between the flexing blade and the metal sheet. Friction in this way dissipates flexural energy. Contact force between the metal sheet insert and the host blade material is provided both by elastic deformation of the inserted metal sheet and by centripetal force which serves to push the metal sheet against the edge and against the bottom of the pocket machined into the blade.
It is an object of the present invention to provide a rotor blade for a turbine engine rotor assembly that includes a means for damping higher order modes of vibration.
It is another object of the present invention to provide means for damping vibration in a rotor blade which minimizes disturbance to air flow adjacent the rotor blade.
It is still another object of the present invention to provide means for damping vibration in a rotor blade which does not negatively affect the structural integrity of the rotor blade.
It is still another object of the present invention to provide means for damping vibration in a rotor blade which can be installed easily and in a cost efficient manner.
It is still another object of the present invention to provide means for damping vibration in a rotor blade that can be tailored and positioned in the blade to counteract specific vibratory conditions.
More specifically, the present application discloses a rotor blade for a turbine engine rotor assembly. The rotor blade comprises an airfoil, having a curvature and a pocket formed in a chordwise surface, the pocket open to the surface; and means for damping vibrations in the blade, the means including a damper where the damper is received within the pocket and the damper is contoured to match the curvature of the airfoil.
In a preferred embodiment of the rotor blade, one or more edges of the pocket are undercut as a means of locating and restricting motion of the damper in the pocket.
In another preferred embodiment, the damper is contoured to be received by the pocket with sufficient contact surface area.
In another preferred embodiment, the damper is retained in the pocket using only frictional forces acting on the contact surface area.
In a more preferred embodiment, centripetal forces acting on the damper tend to retain the damper within the pocket.
The present application also discloses a method for damping vibrations in a rotor blade. The method comprises the steps of determining vibratory characteristics of the rotor blade, including determining where strain energy regions exist for selected modes of vibration within the airfoil; determining a geometry for one or more pockets in a chordwise surface of the airfoil, such that the pockets are located in high strain energy regions of the rotor blade; forming the pocket in the chordwise surface; and inserting a damper in the pocket.
These and other objects, features and advantages of the present invention will become apparent in light of the detail description of the best mode embodiment thereof, as illustrated in the accompanying drawings.